Prevent loader.conf load failure due to unknown console entries

[smh]

Summary

When processing loader.conf if console contained an entry for an unsupported console then cons_set would return an error refusing to set any console.

This has two side effects:

1. Forth would throw a syntax error and stop processing loader.conf at that point.
2. The value of console is ignored.

#1 Means other important loader.conf entries may not be processed, which is clearly undesirable.
#2 Means the users preference for console aren't applied even if they did contain valid options. Now we have support for multi boot paths from a single image e.g. bios and efi mode the console preference needs to deal with the need to set preference for more than one souce.

Fix this by:

1. Returning CMD_OK where possible from cons_set.
2. Allowing set with at least one valid console to proceed.

Test Plan

Check the procesing of console var in loader.conf for:

1. All valid entries = no error, console set
2. At least one valid entry = warning about invalid entry, console set
3. No valid entries = warning about no valid consoles

In all cases the entire of loader.conf should be processed.

[smh]

Remove unused changes from bootstrap.h

[allanjude]

seems like a reasonable solution to me